> commit 4f8f751961b536f77c8f82394963e8e2d26efd84 > Author: Paul E. McKenney <paul...@linux.ibm.com> > Date: Tue Dec 4 14:59:12 2018 -0800 > > torture: Explain and simplify odd "for" loop in mkinitrd.sh > > Why a Bourne-shell "for" loop? And why 192 instances of "a"? This commit > adds a shell comment to present the answer to these mysteries. It also > uses a series of factor-of-four Bourne-shell assignments to make it > easy to see how many instances there are, replacing the earlier wall of > 'a' characters. > > Reported-by: Josh Triplett <j...@joshtriplett.org> > Signed-off-by: Paul E. McKenney <paul...@linux.ibm.com> > > diff --git a/tools/testing/selftests/rcutorture/bin/mkinitrd.sh > b/tools/testing/selftests/rcutorture/bin/mkinitrd.sh > index da298394daa2..ff69190604ea 100755 > --- a/tools/testing/selftests/rcutorture/bin/mkinitrd.sh > +++ b/tools/testing/selftests/rcutorture/bin/mkinitrd.sh > @@ -40,17 +40,24 @@ mkdir $T > cat > $T/init << '__EOF___' > #!/bin/sh > # Run in userspace a few milliseconds every second. This helps to > -# exercise the NO_HZ_FULL portions of RCU. > +# exercise the NO_HZ_FULL portions of RCU. The 192 instances of "a" was > +# empirically shown to give a nice multi-millisecond burst of user-mode > +# execution on a 2GHz CPU, as desired. Modern CPUs will vary from a > +# couple of milliseconds up to perhaps 100 milliseconds, which is an > +# acceptable range. > +# > +# Why not calibrate an exact delay? Because within this initrd, we > +# are restricted to Bourne-shell builtins, which as far as I know do not > +# provide any means of obtaining a fine-grained timestamp. > + > +a4="a a a a" > +a16="$a4 $a4 $a4 $a4" > +a64="$a8 $a8 $a8 $a8"
Mmh, are you sure you don't want s/a8/a16/ here? ;-) Andrea > +a192="$a64 $a64 $a64" > while : > do > q= > - for i in \ > - a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a \ > - a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a \ > - a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a \ > - a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a \ > - a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a \ > - a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a > + for i in $a192 > do > q="$q $i" > done >
